EMBEYONIC AXIS AND NOTOCHOKD-PLATE 



37 



The growth of the blastoderm at first chiefly affects the region behind Hensen's 

 knot. The primitive streak elongates and occupies the pointed end of the oval 

 disc which is now generally called the embryonic shield (fig. 56, b and c). 



The next phase is characterised by a shifting of the maximum growth- activity 

 to the part of the shield in front of Hensen's knot, and the embryonic axis begins 

 to be laid down from before backwards, either by proliferation of the cells at the 

 anterior end of the primitive streak, as some think, or by gradual conversion of 

 the growing streak into the axis, as is indicated by experiments on the developing 

 blastoderm. 1 As the portion of the shield in front of the streak increases in length, 

 the mass of cells extending from Hensen's knot (protochordal process), already 

 described, is necessarily lengthened into a column of cells (figs. 60, 61). It 

 becomes, however, at the same time flattened out into a plate ; and as it is fused 



am emliy. ect. 



y.s. 



FIG. 54. SECTION OF AN EABLY EMBRYO OF SEMNOPITHECUS NASICUS. (After Selenka.) 

 am, amnion; emby, ect., embryonic ectoderm; ?/.s., yolk-sac; ent, entoderm ; mes, mesoderm 

 covering yolk-sac and extending between embryonic ectoderm and entoderm ; con. stk., connecting- 

 stalk mesoderm. 



with the primitive entoderm on its under aspect, the result of this flattening and 

 opening out is, that it comes to roof-in the cavity under the shield along the 

 middle line. This plate is named the notochord- plate , 2 and is recognisable in 

 some cases as a shading in front of the streak in surface views of the blastoderm 

 (fig. 63). A section of the shield (fig. 64) shows that the plate is continuous on 

 each side with the general entoderm, and, where plate and entoderm join, also 

 with wings of mesoderm which spread outwards between ectoderm and entoderm. 

 In front the plate passes into the thickened primitive entodermic plate, and 

 behind into the mass of cells at the head end of the primitive streak, out of 



1 See Assheton, Proc. Hoy. Soc. 1896, also Anat. Anzeiger, xxvii. ; Kopsch, Verhl. d. fiinfte 

 internat. zool. Kongress, Berlin, 1901, and Internat. Monatschr. f. Anat. und Phys. xix. ; Peebles, 

 Archiv f. Entwickelungsmech. vii. 



" It has been also termed the archenteric plate (Urdarmplatte, Bonnet), for reasons which will be 

 given later on. 



